In the prior art, it is known that layers of nematic liquid crystals or of nematic-cholesteric liquid crystal mixtures exhibit electro-optical effects which can be rapidly controlled by subjecting the layer to variations in electrical voltage not exceeding a few tens of volts.
All these effects are based on the common property of mesomorphous materials, when in contact with a solid wall, of orientating the long molecules of which they consist in a common direction, either parallel or perpendicular (the orientation is then called "homeotropic") to the plane of the wall. The direction of this parallel or homeotropic orientation depends on the respective natures of the liquid crystal material and the wall material; it is furthermore greatly facilitated by introducing traces of appropriate surfactants into the mesomorphous material as well as by preliminary treatment of the walls in contact with the film (by rubbing scratching or evaporation at a grazing incidence of a film of silicon oxide, enabling for example the liquid crystal to be orientated parallel to a particular direction of the wall, which is the direction of rubbing or scratching or the projection of the direction of incidence). These layers in uniform orientation are perfectly transparent to visible light. Nematic films are furthermore very highly double-refractive uniaxial media, the optical axis being parallel to the direction of alignment.
The mesomorphous materials elongated molecules exhibit pronounced dielectric anisotropies, and under the action of an electric field, are thus submitted to a torque which tend to arrange them either in parallel or perpendicular orientation to the field according to whether this anisotropy is positive or negative. Consequently, if a layer of a nematic material is enclosed between two transparent electrodes subjected to a variable direct or alternating voltage, it will be found, if the material exhibits a positive dielectric anisotropy and is disposed in parallel orientation, that above a threshold voltage (of the order of 1 to 5 for a layer about ten microns thick) the birefringence of the layer measured perpendicularly to its plane decreases when the applied voltage is increased: the molecules tend to become orientated parallel to the field, and so the optical axis, initially perpendicular to the direction in which the light is propagated, tilts progressively towards a parallel direction. The phenomena are opposite with a homeotropically orientated negative-anisotropy material: the optical axis, initially perpendicular to the layer and parallel to the direction of propagation, tilts towards the plane of the layer, and so the birefringence progressively appears. This effect, called "controlled birefringence", enables the brightness of monochromatic light, or the colour of polychromatic light, transmitted by a nematic layer to be spatially modulated, by imparting local variations to the voltage applied to the layer and arranging the latter between parallel or crossed polarizers.
It is also possible to take advantage of the dielectric and optical anisotropic properties of nematic materials in order to control electrically the light transmitted by a thin layer using the structures called "twisted nematic". A thin layer of positive dielectric anisotropy is inserted in parallel orientation between two transparent electrodes superficially treated, as previously indicated, to orient the molecules of the layer in contact with one wall and the other in two perpendicular directions; the molecules of the intermediate planes in the layer, while remaining parallel to the plane of the walls, rotate progressively about an axis perpendicular to the layer so as to avoid any discontinuity in orientation from one wall to the other. The result of this is a twisted structure which, because of the high optical anisotropy of the material, possesses the advantageous property of imparting rotation through 90.degree. to linearly polarized light being propagated perpendicularly to the strip. If the layer is subjected to a voltage greater than a threshold voltage, the molecules tend to straighten up and to adopt a homeotropic structure, which causes any anisotropy to disappear. When disposed between crossed (or parallel) polarizers, such a layer will totally transmit (or stop) the incident light at the points where it is subjected to a voltage of less than the threshold voltage; at the points where the applied voltage is greater than the threshold voltage the incident light is transmitted to a lesser (or greater) extend as an increase in this voltage causes the structure of the layer to approach more closely to the homeotropic structure.
If a continued increase is maintained in the voltage to which a thin layer in the nematic phase is subjected, a second optical phenomenon which very quickly masks the phenomenon of double refraction appears above a second threshold voltage (of the order of 5 to 7 volts for a layer about ten microns thick): the layer, initially perfectly transparent, acquires a more and more diffusing nature when the applied voltage increases. This effect, which is called "dynamic scattering", and is also applicable to images display is due to the movement under the field action, of electrically charged particles which disorganizes the initially uniformly oriented layer into randomly oriented, and consequently light scattering, clusters. Just like controlled birefringence, dynamic scattering is perfectly reversible. However, adding a small amount of cholesteric material to the nematic material results in a mixture wherein the scattering structure persists for several hours when the control voltage is restored to a value smaller than the dynamic scattering threshold voltage, and consequently having optical storage properties. The application of an alternating voltage at a frequency higher than that of the control voltage enables the mixture to be restored to its initial transparent state.
In order to obtain local variations in the voltage to which the thin layer of liquid crystal is subjected, M. FRAPPIER, G. ASSOULINE, M. HARENG and E. LEIBA proposed, in an article entitled "Liquid-crystal photoconductive image-converter" published in the Nouvelle Revue d'Optique Appliquee (1971, 2, No. 4, pp 221-228) an invention protected by U.S. Pat. No. 3,803,408 issued to G. ASSOULINE, E. LEIBA and E. SPITZ on Apr. 8, 1974., that a thin layer (a few micrometers in thickness) of a photoconductive material, whereof the detection threshold is chosen for example in the near U.V. region, be disposed between one of the transparent electrodes and the nematic material. An image in ultra-violet light, which may be of low intensity, is projected on to the photoconductive layer, whereof it spatially modulates the resistivity. At the unlit points, the nematic layer is subjected to only a fraction of the voltage applied to the electrodes, and remains transparent if matters are so arranged that this fraction of the voltage is at most equal to the diffusion threshold; on the contrary, at the illuminated points the resistance of the photoconductor decreases, the voltage applied to the liquid crystal increases, and the layer acquires a scattering structure. The whole is illuminated with white light which, even if of very high intensity, passes through the photoconductive film without modifying its resistivity if its ultra-violet component has been suitably filtered out. This results in an image-converter which can be used to project T.V. pictures onto a large screen.
In an U.S. Pat. No. 3,798,452 that issued to E. SPITZ, E. LEIBA and G. ASSOULINE on Mar. 19, 1974, it was proposed that the layer of nematic material be replaced by a storage layer consisting of a nematic-cholesteric mixture. It does not then matter what the spectral detection threshold of the photoconductor is, the image being written in with the voltage applied by a low-intensity light-beam, and being projected by a high-intensity beam after the voltage applied to the electrodes has been switched off.
In an U.S. Pat. No. 3,829,684, that issued to G. ASSOULINE, M. HARENG and E. LIEBA on Aug. 13, 1974, it has been furthermore proposed that a fluorescent material layer be deposited on a transparent electrode covering a photoconductive film co-operating with a thin layer of a nematic material or of a nematic-cholesteric mixture, in order to embody a device for visible display of images projected by means of ionizing radiation, the device being more particularly applicable to radioscopy. The light-radiation emitted by the fluorescent screen under the impact of X-radiations for example, by locally modifying the electrical resistance of the photoconductor, spatially modulates the voltage to which the layer of liquid crystal is subjected, which layer thus acquires a more or less scattering nature. This device provides images which are better contrasted, thanks to the dynamic scattering threshold effect, and brighter than those provided by conventional fluorescent screens; furthermore, the use of a liquid crystal mixture having storage properties protects the observer from any radiation, since examination of the image may then be carried out after the ionizing radiation has been switched off.
In these devices, the nature of the photoconductor materials (zinc oxide, cadmium sulphide, selenium) and the fact that they must be used in the form of a thin layer a few micrometers in thickness, involve critical problems in embodiment, due more particularly to the too low transverse electrical resistance of these layers in the absence of illumination. Furthermore, in uses in conjunction with radioscopy, the thickness of the photoconductive layer being too small to absorb the X-radiation sufficiently, the latter must be first of all converted into ultra-violet or visible radiation by way of a fluorescent screen.